3. Climate and the usefulness of green roofs
The green roof is NOT(!) a good solution everywhere.
The original ideology of the invention of green roof is to give back to the
nature the area (on the top of the building), which was taken by the builing
itself.
But there are certain cases, when there was not taken the area, because
originally there was not any vegetation.
These cases: Buildings and settlements in deserts; buildings in high
mountain ranges; buildings in Antarctica and in the Arctic.
There are also not so certain cases (i. e. green roofing in Dubai, and in
similar semi-desert places). Fom this aspect, the existence of all intensive
green roofs can be queried but those, which are irrigated by greywater or
which are food producting areas.
In general the denser is the vegetation, the better solution is the green roof.
Just considering the rainforest sites: using enough thick growing medium,
the same vegetation can be established that was there before.
9. Heat insulation and capacitance
The green roof is a good heat insulator. This is a fact, but the rates can be
(and used to be) queried.
The problem and the advantage at the same time is the water.
The green roof is the only building part where the water is welcome.
Water is a bad heat insulator, furthermore in the temperate zone it can
cause serious damage due to frost.
All conventional heat insulator material is waterproofed. So that their heat
insulation ability can be considered as a constant.
In case of green roofs the heat insulation ability is changing. But as the heat
insulation ability decrease due to the appearance of water the heat
capacitance increases correspondingly.
Greeen roof is excellent in summer and quite good in winter. Due to the
great heat capacitance it is the best solution in regions, where the daily
temperature diference is great, while the yearly is low (mediterranean
zones).
12. Climate effect
As it can be seen from the previous graphs, the temparature extremities are
highly mitigated by the green roofs both on their surfaces and in the
interiors.
Thus, a greater number/area/density of them can have a similar mitigating
effect onto the local climate.
The greatest problem of present day settlements besides the air pollution is
the urban heat island.
Depending on the size, the number of the inhabitants and the building
density the greatest temperature difference of downtowns and the
surrounding natural areas can be even 7 °C.
As it can be seen from the graphs the mitigating effect of green roofs can
remarkably reduce this difference.
A great number of green roofs in downtowns can have such effect.
Nevertheless, altough there are good single examples, there can not be
found such a settlement yet.
13. Urban heat island
The spatial distribution of urban heat island. There can be seen the
mitigating effects of green areas by parks. This is the so called Park Cool
Island (PCI). The temperature here is not so low like in rural zones, but
significantly lower than in the downtown.
14. Urban heat island
The daily periodicity of urban heat island. Considering the daily distribution
of the temperature mitigating effect of geen roofs it can be declared that
these vegetation covered rooftops are excellent tools for decreasing the
negative effects of the urban heat island.
Energydensity
ofradiation
Surfacetemperature
Radiation
surplus
Radiation
shortage
15. Climate effect
An other very important effect is the mitigation ability of heavy rainfalls.
Because of the global climate change heavy rainfalls occur more and more
often. At the same time, the sewage systems of our settlements are not
designed for such sudden impacts.
The green roofs, even the extensive green roofs with their thin growing
medium layers can remarkably mitigate these rainfalls.
On the one hand they can retain a lot of water in the drainage and the
growing medium layers. Naturally the vegetation layer also can retain some.
On the other hand, and from this point of view this is the more important
phenomenon, these roof can mitigate slow down and on this way smooth
these sudden intensive rainfalls.
This means, that may be the 90% of the rain gets into the sewage system,
but not in 15 minutes, rather in 1-2 hours. Thus, the necessary capacitance
of the sewage system is only 10-12% (!) that of the other case.
16. Ecological effects
The green roofs acts as habitat for wildlife.
The extensive green roofs are excellent for insects, birds, and some annual
and perennial plants. These can bear those arid and extreme
circumstances, but do not bear the continuous presence of humans.
The intensive green roofs can play similar role. Theoretically, the irrigation
provide the possibility of greater biodiversity, but the permanent
maintenance, weeding and human presence acts against it.
Even on higher buildings a green roof provides habitat for such species.
Sometimes these are very important, beacuse there exist such species,
which absolutely lost their habitat (in case of great cities) and their last
asylums are such green roofs.
Some of these green roofs are natural protection areas.
There are so called blue roofs, wich are small ponds on the rooftops.
There exist so called brown roofs, where the plants are native and
spontaneously grown (somtimes accelerated by seeding).
17. Ecological effects (disadvantages)
Workers spray pesticide in Yoyogi park, Tokyo. The park had to be closed
because of Dengue fever and infectious mosquitos. A disadvantage comes from
the pairing of global climate change and urban heat island.
In case of green roofs the mosquitos live immediately next to the windows.
18. Blue roof
Oasis 21 Nagoya, Japan. The most extreme example of a blue roof.
19. Brown roof
A brown roof. Very similar to an exensive green roofs, but with native species.
20. Brown roof
A brown roof with a bird nest. Sedum can be native even on brown roofs.
